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Chemical and structural changes in XiMeng lignite and its carbon migration during hydrothermal dewatering
Fuel 148 (2015) 139–144
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Chemical and structural changes in XiMeng lignite and its carbon migration during hydrothermal dewatering Junhong Wu, Jianzhong Liu ⇑, Xu Zhang, Zhihua Wang, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s HTD upgrade mechanism were presented. Aliphatics in lignite decreased whereas aromatics increased after HTD. Oxygen-containing groups in lignite, were significantly removed after HTD. The occurrence and migration mechanism of carbon during HTD were determined.
a r t i c l e
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Article history: Received 9 November 2014 Received in revised form 22 December 2014 Accepted 28 January 2015 Available online 9 February 2015 Keywords: Lignite Hydrothermal dewatering (HTD) Solid-state NMR Chemical structure Carbon migration
a b s t r a c t Changes in the structural features of XiMeng (XM) lignite during hydrothermal dewatering (HTD) were investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy. Gas-, solid-, and liquid-phase products obtained after HTD were also analyzed, and the occurrence of carbon and its migration mechanism during HTD were determined. The equilibrium moisture of XM lignite decreased from 21.44% to 5.73% and oxygen/carbon atomic ratios decreased from 0.25 to 0.14 when HTD was performed at 320 °C. 13C cross-polarization/total suppression of spinning sidebands NMR spectroscopy was used to characterize structural changes in lignite before and after HTD modification. The signal intensities of aliphatics decreased whereas those of aromatics increased after HTD; these findings indicate that coal rank improved after HTD upgrade. Oxygen-containing groups, such as carboxyls, hydroxyls, and methoxyls, significantly decreased in number after HTD. Production of CO2 and CO, the main gaseous products, was attributed to decomposition of carboxyl and carbonyl groups, respectively. Solid matter migrated into gaseous and liquid products at high temperatures, and the carbon-holding capacity of solid products remained as high as 96%. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Coal is the primary source of energy in China [1], and the country is rich in lignite reserves. Unfortunately, inherent moisture, high oxygen contents, and low calorific values greatly restrict the large-scale application of lignite [2]. Lignite features large amounts of hydrophilic functional groups and well-developed pore structures that promote absorption of large quantities of water on its surface. High moisture contents add to transportation costs. Lignite also presents lower energy contents than bituminous coal because of its lower carbon and higher ash contents. Nonetheless, the high reactivity of lignite has attracted increased attention to its use as a feedstock for gasification [3,4]. Thus, upgrading lignite by increasing its calorific value, decreasing its moisture content, and ⇑ Corresponding author. Tel.: +86 571 87952443 5302; fax: +86 571 87952884. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.fuel.2015.01.102 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
removing organic and inorganic elements will be of great help in increasing its energy value. Hydrothermal dewatering (HTD) is a non-evaporative method whereby water is effectively removed and the chemical structure of lignite is changed. HTD is a popular research topic because it endows lignite with excellent upgraded characteristics [5–9]. Continuous efforts to develop HTD are generally directed toward two aspects: (1) the effects of HTD conditions on the characteristic qualities of coal, such as its moisture content, calorific value, and intraparticle porosity, and its use in slurrying, combustion, and gasification and (2) the effects of HTD on wastewater, since many organic compounds can leach out from raw coal to liquid products. According to Favas et al., reaction temperatures mainly affect the intraparticle porosity of HTD products [10]. Yu et al. performed HTD on two Chinese lignites and found that the slurryability of lignite is significantly enhanced by removal of oxygen-containing groups, such as carboxyls and phenolic hydroxyls [11]. In addition,
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large amounts of organic compounds and metal ions were observed to leach out from coal and into liquid products. Thus, efficient treatment of wastewater (i.e. the liquid products) is an urgent issue that must be addressed prior to HTD application. Nakagawa et al. found that organic compounds in wastewater may be completely gasified into CH4 and H2 when it is treated using a novel Ni-supported carbon catalyst [12]. Yu et al. prepared coal–water slurry (CWS) using the liquid and solid products of HTD and found 1–1.5% increases in the fixed-viscosity concentration of CWS; this increase is attributed to ions released into the liquid products that reduce surface tension [13]. Despite these recent gains in HTD knowledge, however, detailed information on the chemical changes in lignite during HTD remains insufficient. Thus, further explorations of the HTD upgrade mechanism are warranted. Analytical methods such as Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance (NMR) spectroscopy have been employed to elucidate several aspects of coal structures. Among these nondestructive techniques, NMR spectroscopy is considered particularly suitable for providing novel structural insights [14]. Cao et al. used solid-state 13C NMR spectroscopy to report chemical structural changes in coal lithotypes before and after high-pressure CO2 injection [15]. Mao et al. further detected similar structural changes in highly volatile bituminous coal using advanced spectral editing techniques [16]. This paper aims to determine the compositions of gas-, solid-, and liquid-phase products obtained through HTD and analyze the migration and distribution of carbon in these products. Detailed structural changes in lignite after HTD treatment are discussed and new insights into the HTD upgrade mechanism are presented.
2. Experimental 2.1. Materials Typical Chinese lignite from XiMeng (XM), Inner Mongolia was used for HTD treatment. The coal sample was milled and sieved through a 150 lm sieve. Moisture content, which is an important coal property, is influenced by environmental temperature and humidity. Both raw and treated coals were exposed to a constant temperature (25 °C) and humidity (65%) to achieve equilibrium, after which the moisture content was obtained. This moisture was considered the equilibrium moisture (Meq).
2.2. HTD procedure As shown in Fig. 1, HTD upgrade was conducted in a 2 L cylindrical autoclave (WHFS-2, Weiba, China) equipped with an automatic temperature controller capable of producing a maximum pressure of 25 MPa and maximum temperature of 350 °C. Lignite (200 g, dry basis) and distilled water (400 g) were placed in the autoclave. The autoclave was pressurized with nitrogen to 3.0 MPa for 2 h to remove air residues and check for air leakage. The nitrogen was then completely released. The reactor was heated to the required temperature (200, 250, 280, 300, or 320 °C) at an average heating rate of 2.5 °C/min. After allowing reaction for 1 h, the autoclave was cooled to room temperature. Gaseous products were released through a gasometer and collected in a reservoir bag. Solid and liquid products were separated using qualitative filter paper. Treated coals were dried in a vacuum oven at 60 °C for 24 h and then labeled HTD-200, HTD-250, HTD-280, HTD-300, and HTD-320 based on the treatment temperature applied.
Fig. 1. Schematic of the hydrothermal reaction system.
2.3. Gaseous product analysis Hydrogen sulfide (H2S) was measured using an electrochemical hydrogen sulfide analyzer (EC-400, Changding, China); other conventional gases were determined using a gas chromatograph (7890A, Agilent, USA). 2.4. Liquid product analysis Metal cations were analyzed by ion-coupled plasma–mass spectroscopy (ICP–MS) (ICAP6000, Thermo Fisher, USA). Anions were measured by an ion chromatograph (ISC-2100, Thermo Fisher, USA). Total organic carbon (TOC) and inorganic carbon (IC) in the liquid products were determined using a TOC-V CPN analyzer (Shimadzu, Japan). 2.5.
13
C NMR spectroscopy
All 13C NMR measurements were performed using an NMR spectrometer (Avance 300, Bruker, Germany) at 75 MHz with a 4 mm triple-resonance probe. Structural information was elucidated by the 13C cross polarization/total sideband suppression (CP/TOSS) technique at a spinning speed of 5 kHz and cross polarization (CP) time of 1 ms; a 1H 90° pulse length of 4 ls and recycle delay of 1 s were also applied. Coal samples were subjected to 2000 scans.
3. Results and discussion 3.1. Effects on coal properties Analysis results of XM lignite properties before and after HTD are shown in Table 1. The Meq content of XM lignite significantly decreased after HTD modification, especially at high treatment temperatures. In particular, Meq decreased from 21.44% in raw coal to 5.73% in HTD-320; this result confirms that inherent moisture is removed irreversibly and that the water-holding capacity of coal decreases after HTD. Carbon contents and calorific values increased whereas volatile matter contents gradually decreased as treatment temperature increased. These results clearly indicate that HTD promotes lignite carbonization and volatile matter decomposition, thereby resulting in enhanced lignite energy density.
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J. Wu et al. / Fuel 148 (2015) 139–144 Table 1 Lignite property analysis results obtained before and after HTD upgrade. Samples
Raw coal HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
Proximate analysis (%)
Qb,eq (MJ/kg)
Meq
Aeq
Vdaf
FCdaf
21.44 15.08 12.57 9.49 7.04 5.73
11.01 11.83 12.55 13.09 13.75 14.18
46.42 44.93 43.43 41.97 40.26 38.89
53.58 55.07 56.57 58.03 59.74 61.11
19.02 20.82 21.88 23.24 24.39 25.35
Ultimate analysis (%) Cdaf
Hdaf
Ndaf
Sdaf
Odaf
69.71 70.35 72.58 74.22 75.98 78.15
5.23 5.05 5.01 5.01 5.06 5.16
1.12 1.20 1.23 1.25 1.29 1.33
0.80 0.78 0.81 0.83 0.78 0.75
23.14 22.62 20.37 18.69 16.89 14.61
Ao/c
AH/C
0.25 0.24 0.21 0.19 0.17 0.14
0.90 0.86 0.83 0.81 0.80 0.79
Note: M refers to moisture content. A, V, and FC respectively refer to ash, volatile, and fixed carbon contents. Qb refers to the bomb calorific value. ‘‘eq’’ refers to equilibrium moisture basis and ‘‘daf’’ refers to dry ash-free basis.
C=O COO O-aro aromatic C O-alkyl
alkyl C
raw coal
HTD-250
Intensity
Ultimate analysis showed that carbon contents increased whereas oxygen contents decreased significantly after HTD upgrade. Decreases in atomic ratios of oxygen to carbon (AO/C) and hydrogen to carbon (AH/C), two parameters related to coal rank, indicate removal of oxygen-containing functional groups and improvements in coal rank. Furthermore, the losses of oxygen are ascribed to the reduction and dehydration reactions [17]. In summary, the coal composition of lignite changed after HTD upgrade via dewatering, oxygen-containing group removal, and volatile matter decomposition. Dewatering is directly related to oxygen-containing group removal. Significant amounts of water are absorbed into or trapped on the pore surface of lignite because of the strong hydrophilic property imparted by the presence of oxygen-containing groups [18–20]. Thus, the water-holding capacity of coal is considerably reduced after removal of these functional groups. It is of interest to note that the coals after HTD modification might be considered to be somewhat analogous to higher-rank coals, and HTD can be regarded as somewhat analogous to the coalification process although considerably accelerated by the temperature and pressure applied [21].
HTD-320
250
200
150
100
50
0
-50
Chemical shift (ppm) Fig. 2.
13
C CP/TOSS spectra of the coal samples before and after HTD upgrade.
3.2. Chemical structural changes during HTD upgrade The structural properties of lignite may be irreversibly changed by breakage of chemical bonds and molecular chains at certain temperatures and saturated vapor pressures. Given the important functions of chemical structures in the reaction mechanism of HTD, semi-quantitative 13C CP/TOSS NMR experiments were performed to characterize structural changes in lignite during HTD upgrade. 13C CP/TOSS NMR is capable of effectively comparing intensity distributions among similar samples under constant conditions (i.e. identical CP times, recycle delays, and scan numbers) [14,22]. The 13C CP/TOSS NMR spectra of the coal samples before and after HTD modification are nearly identical, as shown in Fig. 2. The spectra of the coal samples are mainly composed of two broad bands in the aliphatic (0–90 ppm) and aromatic (90–165 ppm) regions. Functional groups that contributed to the chemical shift ranges in the 13C NMR spectra include alkyl (alk) at 0–50 ppm, O-alkyl (O-alk) at 50–90 ppm, aromatic CAC (Haro) at 90– 150 ppm, aromatic CAO (O-aro) at 150–165 ppm, carboxyls (COO) at 165–190 ppm, and carbonyls (C@O) at 190–220 ppm [16]. The relative contents of specific types of chemical groups in raw coal were determined using spectral editing techniques. As shown in Table 2, the chemical structure of XM lignite consisted of 58.13% aromatics (FHaro + FOaro) and 35.98% aliphatics (Falk + FOalk). Most of the oxygen-containing groups included carboxyls (COO) and hydroxyls (O-alk and O-aro); only 0.91% of these groups was attributed to carbonyls. To investigate changes in the chemical components of the coal samples after HTD, the spectral signals of raw coal were used as a reference. The effects of HTD upgrade on the NMR signal intensities of
different functional groups are shown in Table 3. The main changes observed in the coal spectra included respective decreases and increases in relative percentages of alkyl and aromatic carbons as the HTD temperature increased. In particular, the alkyl carbon signal of HTD-320 was reduced by 80.1% whereas that of aromatic carbons increased by 22.18% compared with the corresponding values of raw coal. These trends may be attributed to desorption of volatile aliphatic hydrocarbons and polymerization of aromatic groups and/ or hydrocarbons [23]. Oxygen-containing group contents significantly decreased with increasing HTD temperature. The intensities of O-alk groups from 50 ppm to 90 ppm, which are assigned to aliphatic hydroxyls, methoxyls, and ethers, initially increased and subsequently decreased as HTD temperature increased, likely because lignite is oxidized by small amounts of residual oxygen trapped in pores at temperatures lower than 250 °C. As the temperature further increased, oxidation decreased and hydrogen bonded hydroxyl groups tended to be destroyed or broken during thermal treating processes [24–26]. The relative low field intensities of carboxyl (165–190 ppm) and carbonyl (190–220 ppm) groups consistently decreased as HTD temperature increased. It indicates a partial decomposition of the unstable carboxyl and carbonyl groups owing to the deep HTD modification. Relative intensities of resonance at around 150–165 ppm, which are assigned to phenolic hydroxyl, only slightly decreased after HTD because of the high bonding energy and stable chemical property of these groups. 3.3. Gaseous product analysis Volatile hydrocarbons in lignite decomposed and large amounts of gaseous products were generated during HTD. As shown in
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Table 2 Relative contents of different functional groups in raw coal. Parameter Chemical shift (ppm)
Falk 0–50
FOalk 50–90
FHaro 90–150
FOaro 150–165
FCOO 165–190
FC=O 190–220
Raw coal
32.63%
3.35%
50.02%
8.11%
4.97%
0.91%
Note: Falk, relative content of alkyl carbon (in % total C).
Table 3 Effect of HTD upgrade on the NMR signal intensities of different functional groups.
a
Samples Chemical shift (ppm)
alk (%) 0–50
O-alk (%) 50–90
Haro (%) 90–150
O-aro (%) 150–165
COO (%) 165–190
C@O (%) 190–220
Raw coal HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
100a 97.32 96.69 86.16 83.61 80.10
100 112.57 112.74 72.15 65.03 39.18
100 103.55 104.50 114.48 117.33 122.18
100 98.39 98.53 98.52 96.86 94.01
100 82.96 80.30 77.60 71.23 68.27
100 62.23 44.91 38.54 49.59 45.09
(1 0 0)-intensity changes with respect to the peak intensity of XM raw coal.
Table 4, CO2, which made up about 73.03% of the gaseous products in HTD-320, was the most abundant product; such production is attributed to CO2 release from coal micropores and CO2 formation from decarboxylation [27]. Increased temperatures accelerate decarboxylation, which results in higher CO2 concentrations. The amounts of CO2 produced at high temperatures are consistent with the loss of carboxyl groups, as shown in Table 3. Removal of carboxyl groups from the coal structure resulted in production of significant amounts of CO2. CO yield, which initially increased and then slightly decreased with increasing HTD temperature, was lower than CO2 yield. CO began to be produced at low temperature, might be the product of thermal decomposition of carbonyl groups or ethers in the coal [28]. At treatment temperatures lower than 280 °C, only small amounts of CO were gradually generated; production of this gas reduced the carbonyl content of lignite (Table 3). As the HTD temperature further increased, CO concentrations only slightly changed because of the stability of carbonyl groups, but also possibly as the result of a water–gas reaction [7]. These results show that CO release is highly correlated with decarbonylation. Organic gases (CnHm; e.g. CH4, C2H6, and C2H4) were derived from the breakage of alkyl side chains, from the aromatic nuclear structure [29]. The concentration of CnHm increased as HTD temperature increased. In particular, a maximum CnHm percentage of 2.52% was obtained from HTD-320; this result indicates that higher treatment temperatures promote volatile hydrocarbon decomposition and organic gas release, which coincides well with the experimental NMR results. H2S is the major sulfur gas evolved during HTD treatment. H2S gas concentrations increased drastically from 765 ppm in HTD-200 to 12,403 ppm in HTD-320. H2S formation is mainly attributed to decomposition of thermally unstable aliphatic sulfur groups. Thus, HTD positively contributes to lignite desulfurization. The small amounts of O2 obtained are probably due to the residues in the coal pores. Increases in H2 concentration as the HTD temperature increased may be related to water–gas shifting reactions [30].
Proportions of N2, which originates from initial atmospheric nitrogen in the reactor, gradually decreased as gas products were produced during HTD. 3.4. Liquid product analysis Liquid products contain dissolved inorganic minerals and organic compounds. The main ions produced in the liquid products are presented in Table 5. The concentrations of metal cations, including Na+, K+, Ca2+, and Mg2+, increased as temperature increased. Of these cations, Na+ showed the highest concentration (821.75–1434.5 mg/L). Among the anions, SO24 showed the highest concentration (164–570 mg/L); concentrations of this anion decreased, however, as HTD temperature increased. These results indicate that some sulfur in coal is partially leached out in the form of sulfates during HTD. Sulfur elimination is advantageous to reducing pollutants in lignite and allows further utilization of the material. Concentrations of other anions, such as NO3 , Cl , and PO24 , largely varied in a manner independent of HTD temperature. Total organic carbon (TOC) is the amount of carbon bound in an organic compound and is often used as a non-specific indicator of water quality. Inorganic carbon (IC) represents the content of dissolved carbon dioxide and carbonic acid salts. As shown in Fig. 3, TOC concentrations in the liquid products increased as HTD temperature increased; by contrast, evident changes in IC concentrations were not observed. The TOC content in HTD-320 was considerably higher (5350 mg/L) than that in HTD-200 (615 mg/L). The majority of carbons dissolved in wastewater were obtained in the form of organic compounds, which indicates that high temperatures promote breakage of aliphatic chains and leaching of organic compounds from coal to wastewater. 3.5. Mass distributions of three-phase products The mass distribution and chemical composition of HTD products have attracted considerable attention in industrial
Table 4 Analysis of gaseous products after HTD upgrade. Samples
CO2 (%)
O2 (%)
N2 (%)
CO (%)
H2 (%)
CnHm (%)
H2S (ppm)
HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
40.45 53.19 68.00 71.53 73.03
0.74 0.46 0.45 0.45 0.43
57.92 44.28 28.41 23.33 21.64
0.68 1.50 1.93 1.66 1.60
0.09 0.10 0.26 0.46 0.77
0.11 0.46 0.94 1.55 2.52
765 3580 7110 9682 12,403
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J. Wu et al. / Fuel 148 (2015) 139–144 Table 5 Main cation and anion components of HTD liquid products. Samples
K+ (mg/L)
Na+ (mg/L)
Ca2+ (mg/L)
Mg2+ (mg/L)
SO24 (mg/L)
NO3 (mg/L)
Cl (mg/L)
PO34 (mg/L)
HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
36.9 53.1 74.25 78.65 86.35
821.75 966.25 1364.5 1421.8 1434.5
42.73 60.23 323.5 432.0 761.5
98.03 122.35 124.03 127.50 133.50
570 231 216 176 164
14.0 42.5 61.5 73.0 71.5
11.0 14.0 11.5 13.0 13.5
7.5 6.0 10.0 6.5 7.0
3000 2000
3.0 2.5 2.0
96 1.5 94 1.0
percentage (%)
C-solid C-gas C-liquid
98
4000
percentage (%)
Concentration (mg/L)
100
TOC IC
5000
1000
92 0
200
250
280
300
320
0.5 0.0
90
Temperature (°C )
200 °C
250 °C
280 °C
300 °C
320 °C
temperature
Fig. 3. TOC and IC analysis of HTD liquid products.
Fig. 5. Total carbon balance of solid, liquid, and gaseous products after HTD.
applications. The feed materials included 200 g of raw coal (dry basis; 33.33% of the feed) and 400 g of distilled water (67.67% of the feed). After HTD, the solid and liquid products were separated and then weighed. The treated coals were also dried and weighed, and the mass of gaseous products was determined from the weight difference of feed and the total of solid and liquid products. The mass distribution of solid, liquid, and gaseous products after HTD is shown in Fig. 4. The mass of liquid and gaseous products gradually increased whereas that of solid products decreased as HTD temperature increased. These findings suggest that a fraction of the solid products transforms into liquid or gas products with increasing treatment temperature. The percentage of solid products in HTD-320 was only 27.89%; this value represents a drastic decrease of 16.32% compared with the corresponding value in raw coal. By contrast, the percentages of gaseous and liquid products increased by 1.76% and 3.68%, respectively, compared with baseline values. As previously described, increases in the percentage of released gases, such as CO2 and CH4, may be attributed to the breakdown of
600
liquid gas solid
Mass distribution (g)
500 400 66.67% 67.14% 67.88% 68.37% 69.08% 70.35%
aliphatic hydrocarbons. Increases in the percentage of liquid products may be attributed to leaching of soluble salts and TOC from raw coal. 3.6. Carbon migration during HTD Carbon is the main element in coal that provides the majority of its calorific heat. Given that aliphatic carbons are thermally unstable and form small organic molecules and inorganic gases upon heating, HTD modification causes carbon migration from coal to liquid and solid products. The processing temperature exerts an important influence on carbon migration, as shown in Fig. 5. High temperatures caused significant losses in solid carbon, which leads to increased percentages of gaseous and liquid carbons. In HTD200, for example, minimal amounts of carbon were released into the gas (0.41%) and liquid (0.13%) phases. By comparison, higher temperatures accelerated the decomposition of unstable hydrocarbons in lignite, resulting in 3.5% solid carbon reduction in HTD-320 in contrast to increases of 2.2% and 1.3% in gaseous and liquid carbons, respectively. Carbon contents in solid products remained above 96% within HTD temperatures of 200–320 °C, which indicates that preliminary pyrolysis mildly occurs during HTD upgrade. During hydrolysis, the majority of the carbon is retained and lignite is upgraded by the removal of oxygen-containing groups and volatile hydrocarbons. Disposal of CO2 and treatment of TOCs in wastewater are two issues worthy of further discussion.
300
4. Conclusions 200
0
0.57%
0.80%
1.32%
1.63%
1.76%
100 33.33% 32.29% 31.33% 30.31% 29.29% 27.89% 0
Feed materials HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
Fig. 4. Mass distribution of solid, liquid, and gaseous phase before and after HTD.
HTD, a promising upgrade method for low-rank coals, has recently attracted increased research attention. After HTD modification, significant changes in the properties of XM lignite were observed; these changes included removal inherent water, increases in fixed carbon content, decreases in volatile matter content, and removal of oxygen-containing groups. Such changes resulted in improvements in coal rank.
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C CP/TOSS NMR spectroscopy was used to semi-quantitatively characterize structural changes in lignite before and after HTD modification. Chemical structural changes in lignite agreed well with the properties of the HTD products. The signal intensity of aliphatics decreased whereas that of aromatics increased after HTD. The amounts of oxygen-containing groups, such as carboxyls, hydroxyls, and methoxyls, significantly decreased after HTD, and CO2 and CO comprised the majority of gaseous products formed. Formation of these gases is attributed to decomposition of carboxyl and carbonyl groups, respectively. Evident changes in phenolic hydroxyl content were not observed because of the high bonding energy of this functional group. Solid, liquid, and gaseous products obtained after HTD modification illustrated the mass distribution of carbon: solid matter migrated into the gaseous and liquid products. This phenomenon was especially notable at high temperatures. The proportions of gaseous, liquid, and solid products in HTD-320 were 2.2%, 1.3%, and 96.5%, respectively. The carbon-holding capacity of solid products was as high as 96%. In summary, preliminary pyrolysis mildly occurs during HTD; during this process, carbon contents are maintained and lignite is mainly upgraded by removal of oxygen-containing groups and volatile hydrocarbons. Thus, HTD is an effective method for upgrading low-rank coals extensively. Acknowledgment The authors wish to acknowledge the financial support provided by the National Basic Research Program of China (Grant No. 2010CB227001). References [1] Li WD, Li WF, Liu HF. The resource utilization of algae-preparing coal slurry with algae. Fuel 2010;89:965–70. [2] Katalambula H, Gupta R. Low-grade coals: a review of some prospective upgrading technologies. Energy Fuels 2009;23:3392–405. [3] Fei J, Zhang J, Wang F, Wang J. Synergistic effects on co-pyrolysis of lignite and high-sulfur swelling coal. J Anal Appl Pyrol 2012;95:61–7. [4] Zhan X, Jia J, Zhou Z, Wang F. Influence of blending methods on the cogasification reactivity of petroleum coke and lignite. Energy Convers Manage 2011;52:1810–4. [5] Favas G, Jackson WR. Hydrothermal dewatering of lower rank coals. 2. Effects of coal characteristics for a range of Australian and international coals. Fuel 2003;82:59–69. [6] Favas G, Jackson WR, Marshall M. Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals. Fuel 2003;82:71–9. [7] Mursito AT, Hirajima T, Sasaki K, Kumagai S. The effect of hydrothermal dewatering of Pontianak tropical peat on organics in wastewater and gaseous products. Fuel 2010;89:3934–42. [8] Racovalis L, Hobday MD, Hodges S. Effect of processing conditions on organics in wastewater from hydrothermal dewatering of low-rank coal. Fuel 2002;81:1369–78.
[9] Sakaguchi M, Laursen K, Nakagawa H, Miura K. Hydrothermal upgrading of Loy Yang Brown coal – Effect of upgrading conditions on the characteristics of the products. Fuel Process Technol 2008;89:391–6. [10] Favas G, Jackson WR. Hydrothermal dewatering of lower rank coals. 1. Effects of process conditions on the properties of dried product. Fuel 2003;82:53–7. [11] Yu YJ, Liu JZ, Wang RK, Zhou JH, Cen KF. Effect of hydrothermal dewatering on the slurryability of brown coals. Energy Convers Manage 2012;57:8–12. [12] Nakagawa H, Namba A, Bohlmann M, Miura K. Hydrothermal dewatering of brown coal and catalytic hydrothermal gasification of the organic compounds dissolving in the water using a novel Ni/carbon catalyst. Fuel 2004;83:719–25. [13] Yu YJ, Liu JZ, Cen KF. Properties of coal water slurry prepared with the solid and liquid products of hydrothermal dewatering of brown coal. Ind Eng Chem Res 2014;53:4511–7. [14] Mao JD, Fang XW, Lan YQ, Schimmelmann A, Mastalerz M, Xu L, et al. Chemical and nanometer-scale structure of kerogen and its change during thermal maturation investigated by advanced solid-state C-13 NMR spectroscopy. Geochim Cosmochim Acta 2010;74:2110–27. [15] Cao X, Mastalerz M, Chappell MA, Miller LF, Li Y, Mao J. Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state C-13 nuclear magnetic resonance spectroscopy. Int J Coal Geol 2011;88:67–74. [16] Mao JD, Schimmelmann A, Mastalerz M, Hatcher PG, Li Y. Structural features of a bituminous coal and their changes during low-temperature oxidation and loss of volatiles investigated by advanced solid-state NMR spectroscopy. Energy Fuels 2010;24:2536–44. [17] Mursito AT, Hirajima T, Sasaki K. Upgrading and dewatering of raw tropical peat by hydrothermal treatment. Fuel 2010;89:635–41. [18] Li WD, Li WF, Liu HF. Effects of sewage sludge on rheological characteristics of coal–water slurry. Fuel 2010;89:2505–10. [19] Liu M, Duan YF, Li HF. Effect of modified sludge on the rheological properties and co-slurry mechanism of petroleum coke–sludge slurry. Powder Technol 2013;243:18–26. [20] Ma XY, Duan YF, Liu M. Effects of petrochemical sludge on the slurry-ability of coke water slurry. Exp Therm Fluid Sci 2013;48:238–44. [21] Fei Y, Artanto Y, Giroux L, Marshall M, Jackson WR, MacPhee JA, et al. Comparison of some physico-chemical properties of victorian lignite dewatered under non-evaporative conditions. Fuel 2006;85:1987–91. [22] Erdenetsogt BO, Lee I, Lee SK, Ko YJ, Bat-Erdene D. Solid-state C-13 CP/MAS NMR study of Baganuur coal, Mongolia: oxygen-loss during coalification from lignite to subbituminous rank. Int J Coal Geol 2010;82:37–44. [23] Lin X, Wang C, Ideta K, Miyawaki J, Nishiyama Y, Wang Y, et al. Insights into the functional group transformation of a Chinese brown coal during slow pyrolysis by combining various experiments. Fuel 2014;118:257–64. [24] Tahmasebi A, Yu J, Han Y, Li X. A study of chemical structure changes of Chinese lignite during fluidized-bed drying in nitrogen and air. Fuel Process Technol 2012;101:85–93. [25] Tahmasebi A, Yu J, Bhattacharya S. Chemical structure changes accompanying fluidized-bed drying of Victorian brown coal superheated steam, nitrogen and hot air. Energy Fuels 2012;26:3651–60. [26] Shui H, Wang Z, Wang G. Effect of hydrothermal treatment on the extraction of coal in the CS2/NMP mixed solvent. Fuel 2006;85:1798–802. [27] Blazso M, Jakab E, Vargha A, Szekely T, Zoebel H, Klare H, et al. The effect of hydrothermal treatment on a Merseburg lignite. Fuel 1986;65:337–41. [28] Jakab E, Blazsó M. Thermal degradation of polyquinones used as coal models. J Anal Appl Pyrol 1983;5:139–49. [29] Yan J, Bai Z, Bai J, Guo Z, Li W. Effects of organic solvent treatment on the chemical structure and pyrolysis reactivity of brown coal. Fuel 2014;128:39–45. [30] Mursito AT, Hirajima T, Sasaki K. Alkaline hydrothermal de-ashing and desulfurization of low quality coal and its application to hydrogen-rich gas generation. Energy Convers Manage 2011;52:762–9.
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Chemical and structural changes in XiMeng lignite and its carbon migration during hydrothermal dewatering Junhong Wu, Jianzhong Liu ⇑, Xu Zhang, Zhihua Wang, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s HTD upgrade mechanism were presented. Aliphatics in lignite decreased whereas aromatics increased after HTD. Oxygen-containing groups in lignite, were significantly removed after HTD. The occurrence and migration mechanism of carbon during HTD were determined.
a r t i c l e
i n f o
Article history: Received 9 November 2014 Received in revised form 22 December 2014 Accepted 28 January 2015 Available online 9 February 2015 Keywords: Lignite Hydrothermal dewatering (HTD) Solid-state NMR Chemical structure Carbon migration
a b s t r a c t Changes in the structural features of XiMeng (XM) lignite during hydrothermal dewatering (HTD) were investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy. Gas-, solid-, and liquid-phase products obtained after HTD were also analyzed, and the occurrence of carbon and its migration mechanism during HTD were determined. The equilibrium moisture of XM lignite decreased from 21.44% to 5.73% and oxygen/carbon atomic ratios decreased from 0.25 to 0.14 when HTD was performed at 320 °C. 13C cross-polarization/total suppression of spinning sidebands NMR spectroscopy was used to characterize structural changes in lignite before and after HTD modification. The signal intensities of aliphatics decreased whereas those of aromatics increased after HTD; these findings indicate that coal rank improved after HTD upgrade. Oxygen-containing groups, such as carboxyls, hydroxyls, and methoxyls, significantly decreased in number after HTD. Production of CO2 and CO, the main gaseous products, was attributed to decomposition of carboxyl and carbonyl groups, respectively. Solid matter migrated into gaseous and liquid products at high temperatures, and the carbon-holding capacity of solid products remained as high as 96%. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Coal is the primary source of energy in China [1], and the country is rich in lignite reserves. Unfortunately, inherent moisture, high oxygen contents, and low calorific values greatly restrict the large-scale application of lignite [2]. Lignite features large amounts of hydrophilic functional groups and well-developed pore structures that promote absorption of large quantities of water on its surface. High moisture contents add to transportation costs. Lignite also presents lower energy contents than bituminous coal because of its lower carbon and higher ash contents. Nonetheless, the high reactivity of lignite has attracted increased attention to its use as a feedstock for gasification [3,4]. Thus, upgrading lignite by increasing its calorific value, decreasing its moisture content, and ⇑ Corresponding author. Tel.: +86 571 87952443 5302; fax: +86 571 87952884. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.fuel.2015.01.102 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
removing organic and inorganic elements will be of great help in increasing its energy value. Hydrothermal dewatering (HTD) is a non-evaporative method whereby water is effectively removed and the chemical structure of lignite is changed. HTD is a popular research topic because it endows lignite with excellent upgraded characteristics [5–9]. Continuous efforts to develop HTD are generally directed toward two aspects: (1) the effects of HTD conditions on the characteristic qualities of coal, such as its moisture content, calorific value, and intraparticle porosity, and its use in slurrying, combustion, and gasification and (2) the effects of HTD on wastewater, since many organic compounds can leach out from raw coal to liquid products. According to Favas et al., reaction temperatures mainly affect the intraparticle porosity of HTD products [10]. Yu et al. performed HTD on two Chinese lignites and found that the slurryability of lignite is significantly enhanced by removal of oxygen-containing groups, such as carboxyls and phenolic hydroxyls [11]. In addition,
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large amounts of organic compounds and metal ions were observed to leach out from coal and into liquid products. Thus, efficient treatment of wastewater (i.e. the liquid products) is an urgent issue that must be addressed prior to HTD application. Nakagawa et al. found that organic compounds in wastewater may be completely gasified into CH4 and H2 when it is treated using a novel Ni-supported carbon catalyst [12]. Yu et al. prepared coal–water slurry (CWS) using the liquid and solid products of HTD and found 1–1.5% increases in the fixed-viscosity concentration of CWS; this increase is attributed to ions released into the liquid products that reduce surface tension [13]. Despite these recent gains in HTD knowledge, however, detailed information on the chemical changes in lignite during HTD remains insufficient. Thus, further explorations of the HTD upgrade mechanism are warranted. Analytical methods such as Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance (NMR) spectroscopy have been employed to elucidate several aspects of coal structures. Among these nondestructive techniques, NMR spectroscopy is considered particularly suitable for providing novel structural insights [14]. Cao et al. used solid-state 13C NMR spectroscopy to report chemical structural changes in coal lithotypes before and after high-pressure CO2 injection [15]. Mao et al. further detected similar structural changes in highly volatile bituminous coal using advanced spectral editing techniques [16]. This paper aims to determine the compositions of gas-, solid-, and liquid-phase products obtained through HTD and analyze the migration and distribution of carbon in these products. Detailed structural changes in lignite after HTD treatment are discussed and new insights into the HTD upgrade mechanism are presented.
2. Experimental 2.1. Materials Typical Chinese lignite from XiMeng (XM), Inner Mongolia was used for HTD treatment. The coal sample was milled and sieved through a 150 lm sieve. Moisture content, which is an important coal property, is influenced by environmental temperature and humidity. Both raw and treated coals were exposed to a constant temperature (25 °C) and humidity (65%) to achieve equilibrium, after which the moisture content was obtained. This moisture was considered the equilibrium moisture (Meq).
2.2. HTD procedure As shown in Fig. 1, HTD upgrade was conducted in a 2 L cylindrical autoclave (WHFS-2, Weiba, China) equipped with an automatic temperature controller capable of producing a maximum pressure of 25 MPa and maximum temperature of 350 °C. Lignite (200 g, dry basis) and distilled water (400 g) were placed in the autoclave. The autoclave was pressurized with nitrogen to 3.0 MPa for 2 h to remove air residues and check for air leakage. The nitrogen was then completely released. The reactor was heated to the required temperature (200, 250, 280, 300, or 320 °C) at an average heating rate of 2.5 °C/min. After allowing reaction for 1 h, the autoclave was cooled to room temperature. Gaseous products were released through a gasometer and collected in a reservoir bag. Solid and liquid products were separated using qualitative filter paper. Treated coals were dried in a vacuum oven at 60 °C for 24 h and then labeled HTD-200, HTD-250, HTD-280, HTD-300, and HTD-320 based on the treatment temperature applied.
Fig. 1. Schematic of the hydrothermal reaction system.
2.3. Gaseous product analysis Hydrogen sulfide (H2S) was measured using an electrochemical hydrogen sulfide analyzer (EC-400, Changding, China); other conventional gases were determined using a gas chromatograph (7890A, Agilent, USA). 2.4. Liquid product analysis Metal cations were analyzed by ion-coupled plasma–mass spectroscopy (ICP–MS) (ICAP6000, Thermo Fisher, USA). Anions were measured by an ion chromatograph (ISC-2100, Thermo Fisher, USA). Total organic carbon (TOC) and inorganic carbon (IC) in the liquid products were determined using a TOC-V CPN analyzer (Shimadzu, Japan). 2.5.
13
C NMR spectroscopy
All 13C NMR measurements were performed using an NMR spectrometer (Avance 300, Bruker, Germany) at 75 MHz with a 4 mm triple-resonance probe. Structural information was elucidated by the 13C cross polarization/total sideband suppression (CP/TOSS) technique at a spinning speed of 5 kHz and cross polarization (CP) time of 1 ms; a 1H 90° pulse length of 4 ls and recycle delay of 1 s were also applied. Coal samples were subjected to 2000 scans.
3. Results and discussion 3.1. Effects on coal properties Analysis results of XM lignite properties before and after HTD are shown in Table 1. The Meq content of XM lignite significantly decreased after HTD modification, especially at high treatment temperatures. In particular, Meq decreased from 21.44% in raw coal to 5.73% in HTD-320; this result confirms that inherent moisture is removed irreversibly and that the water-holding capacity of coal decreases after HTD. Carbon contents and calorific values increased whereas volatile matter contents gradually decreased as treatment temperature increased. These results clearly indicate that HTD promotes lignite carbonization and volatile matter decomposition, thereby resulting in enhanced lignite energy density.
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J. Wu et al. / Fuel 148 (2015) 139–144 Table 1 Lignite property analysis results obtained before and after HTD upgrade. Samples
Raw coal HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
Proximate analysis (%)
Qb,eq (MJ/kg)
Meq
Aeq
Vdaf
FCdaf
21.44 15.08 12.57 9.49 7.04 5.73
11.01 11.83 12.55 13.09 13.75 14.18
46.42 44.93 43.43 41.97 40.26 38.89
53.58 55.07 56.57 58.03 59.74 61.11
19.02 20.82 21.88 23.24 24.39 25.35
Ultimate analysis (%) Cdaf
Hdaf
Ndaf
Sdaf
Odaf
69.71 70.35 72.58 74.22 75.98 78.15
5.23 5.05 5.01 5.01 5.06 5.16
1.12 1.20 1.23 1.25 1.29 1.33
0.80 0.78 0.81 0.83 0.78 0.75
23.14 22.62 20.37 18.69 16.89 14.61
Ao/c
AH/C
0.25 0.24 0.21 0.19 0.17 0.14
0.90 0.86 0.83 0.81 0.80 0.79
Note: M refers to moisture content. A, V, and FC respectively refer to ash, volatile, and fixed carbon contents. Qb refers to the bomb calorific value. ‘‘eq’’ refers to equilibrium moisture basis and ‘‘daf’’ refers to dry ash-free basis.
C=O COO O-aro aromatic C O-alkyl
alkyl C
raw coal
HTD-250
Intensity
Ultimate analysis showed that carbon contents increased whereas oxygen contents decreased significantly after HTD upgrade. Decreases in atomic ratios of oxygen to carbon (AO/C) and hydrogen to carbon (AH/C), two parameters related to coal rank, indicate removal of oxygen-containing functional groups and improvements in coal rank. Furthermore, the losses of oxygen are ascribed to the reduction and dehydration reactions [17]. In summary, the coal composition of lignite changed after HTD upgrade via dewatering, oxygen-containing group removal, and volatile matter decomposition. Dewatering is directly related to oxygen-containing group removal. Significant amounts of water are absorbed into or trapped on the pore surface of lignite because of the strong hydrophilic property imparted by the presence of oxygen-containing groups [18–20]. Thus, the water-holding capacity of coal is considerably reduced after removal of these functional groups. It is of interest to note that the coals after HTD modification might be considered to be somewhat analogous to higher-rank coals, and HTD can be regarded as somewhat analogous to the coalification process although considerably accelerated by the temperature and pressure applied [21].
HTD-320
250
200
150
100
50
0
-50
Chemical shift (ppm) Fig. 2.
13
C CP/TOSS spectra of the coal samples before and after HTD upgrade.
3.2. Chemical structural changes during HTD upgrade The structural properties of lignite may be irreversibly changed by breakage of chemical bonds and molecular chains at certain temperatures and saturated vapor pressures. Given the important functions of chemical structures in the reaction mechanism of HTD, semi-quantitative 13C CP/TOSS NMR experiments were performed to characterize structural changes in lignite during HTD upgrade. 13C CP/TOSS NMR is capable of effectively comparing intensity distributions among similar samples under constant conditions (i.e. identical CP times, recycle delays, and scan numbers) [14,22]. The 13C CP/TOSS NMR spectra of the coal samples before and after HTD modification are nearly identical, as shown in Fig. 2. The spectra of the coal samples are mainly composed of two broad bands in the aliphatic (0–90 ppm) and aromatic (90–165 ppm) regions. Functional groups that contributed to the chemical shift ranges in the 13C NMR spectra include alkyl (alk) at 0–50 ppm, O-alkyl (O-alk) at 50–90 ppm, aromatic CAC (Haro) at 90– 150 ppm, aromatic CAO (O-aro) at 150–165 ppm, carboxyls (COO) at 165–190 ppm, and carbonyls (C@O) at 190–220 ppm [16]. The relative contents of specific types of chemical groups in raw coal were determined using spectral editing techniques. As shown in Table 2, the chemical structure of XM lignite consisted of 58.13% aromatics (FHaro + FOaro) and 35.98% aliphatics (Falk + FOalk). Most of the oxygen-containing groups included carboxyls (COO) and hydroxyls (O-alk and O-aro); only 0.91% of these groups was attributed to carbonyls. To investigate changes in the chemical components of the coal samples after HTD, the spectral signals of raw coal were used as a reference. The effects of HTD upgrade on the NMR signal intensities of
different functional groups are shown in Table 3. The main changes observed in the coal spectra included respective decreases and increases in relative percentages of alkyl and aromatic carbons as the HTD temperature increased. In particular, the alkyl carbon signal of HTD-320 was reduced by 80.1% whereas that of aromatic carbons increased by 22.18% compared with the corresponding values of raw coal. These trends may be attributed to desorption of volatile aliphatic hydrocarbons and polymerization of aromatic groups and/ or hydrocarbons [23]. Oxygen-containing group contents significantly decreased with increasing HTD temperature. The intensities of O-alk groups from 50 ppm to 90 ppm, which are assigned to aliphatic hydroxyls, methoxyls, and ethers, initially increased and subsequently decreased as HTD temperature increased, likely because lignite is oxidized by small amounts of residual oxygen trapped in pores at temperatures lower than 250 °C. As the temperature further increased, oxidation decreased and hydrogen bonded hydroxyl groups tended to be destroyed or broken during thermal treating processes [24–26]. The relative low field intensities of carboxyl (165–190 ppm) and carbonyl (190–220 ppm) groups consistently decreased as HTD temperature increased. It indicates a partial decomposition of the unstable carboxyl and carbonyl groups owing to the deep HTD modification. Relative intensities of resonance at around 150–165 ppm, which are assigned to phenolic hydroxyl, only slightly decreased after HTD because of the high bonding energy and stable chemical property of these groups. 3.3. Gaseous product analysis Volatile hydrocarbons in lignite decomposed and large amounts of gaseous products were generated during HTD. As shown in
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Table 2 Relative contents of different functional groups in raw coal. Parameter Chemical shift (ppm)
Falk 0–50
FOalk 50–90
FHaro 90–150
FOaro 150–165
FCOO 165–190
FC=O 190–220
Raw coal
32.63%
3.35%
50.02%
8.11%
4.97%
0.91%
Note: Falk, relative content of alkyl carbon (in % total C).
Table 3 Effect of HTD upgrade on the NMR signal intensities of different functional groups.
a
Samples Chemical shift (ppm)
alk (%) 0–50
O-alk (%) 50–90
Haro (%) 90–150
O-aro (%) 150–165
COO (%) 165–190
C@O (%) 190–220
Raw coal HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
100a 97.32 96.69 86.16 83.61 80.10
100 112.57 112.74 72.15 65.03 39.18
100 103.55 104.50 114.48 117.33 122.18
100 98.39 98.53 98.52 96.86 94.01
100 82.96 80.30 77.60 71.23 68.27
100 62.23 44.91 38.54 49.59 45.09
(1 0 0)-intensity changes with respect to the peak intensity of XM raw coal.
Table 4, CO2, which made up about 73.03% of the gaseous products in HTD-320, was the most abundant product; such production is attributed to CO2 release from coal micropores and CO2 formation from decarboxylation [27]. Increased temperatures accelerate decarboxylation, which results in higher CO2 concentrations. The amounts of CO2 produced at high temperatures are consistent with the loss of carboxyl groups, as shown in Table 3. Removal of carboxyl groups from the coal structure resulted in production of significant amounts of CO2. CO yield, which initially increased and then slightly decreased with increasing HTD temperature, was lower than CO2 yield. CO began to be produced at low temperature, might be the product of thermal decomposition of carbonyl groups or ethers in the coal [28]. At treatment temperatures lower than 280 °C, only small amounts of CO were gradually generated; production of this gas reduced the carbonyl content of lignite (Table 3). As the HTD temperature further increased, CO concentrations only slightly changed because of the stability of carbonyl groups, but also possibly as the result of a water–gas reaction [7]. These results show that CO release is highly correlated with decarbonylation. Organic gases (CnHm; e.g. CH4, C2H6, and C2H4) were derived from the breakage of alkyl side chains, from the aromatic nuclear structure [29]. The concentration of CnHm increased as HTD temperature increased. In particular, a maximum CnHm percentage of 2.52% was obtained from HTD-320; this result indicates that higher treatment temperatures promote volatile hydrocarbon decomposition and organic gas release, which coincides well with the experimental NMR results. H2S is the major sulfur gas evolved during HTD treatment. H2S gas concentrations increased drastically from 765 ppm in HTD-200 to 12,403 ppm in HTD-320. H2S formation is mainly attributed to decomposition of thermally unstable aliphatic sulfur groups. Thus, HTD positively contributes to lignite desulfurization. The small amounts of O2 obtained are probably due to the residues in the coal pores. Increases in H2 concentration as the HTD temperature increased may be related to water–gas shifting reactions [30].
Proportions of N2, which originates from initial atmospheric nitrogen in the reactor, gradually decreased as gas products were produced during HTD. 3.4. Liquid product analysis Liquid products contain dissolved inorganic minerals and organic compounds. The main ions produced in the liquid products are presented in Table 5. The concentrations of metal cations, including Na+, K+, Ca2+, and Mg2+, increased as temperature increased. Of these cations, Na+ showed the highest concentration (821.75–1434.5 mg/L). Among the anions, SO24 showed the highest concentration (164–570 mg/L); concentrations of this anion decreased, however, as HTD temperature increased. These results indicate that some sulfur in coal is partially leached out in the form of sulfates during HTD. Sulfur elimination is advantageous to reducing pollutants in lignite and allows further utilization of the material. Concentrations of other anions, such as NO3 , Cl , and PO24 , largely varied in a manner independent of HTD temperature. Total organic carbon (TOC) is the amount of carbon bound in an organic compound and is often used as a non-specific indicator of water quality. Inorganic carbon (IC) represents the content of dissolved carbon dioxide and carbonic acid salts. As shown in Fig. 3, TOC concentrations in the liquid products increased as HTD temperature increased; by contrast, evident changes in IC concentrations were not observed. The TOC content in HTD-320 was considerably higher (5350 mg/L) than that in HTD-200 (615 mg/L). The majority of carbons dissolved in wastewater were obtained in the form of organic compounds, which indicates that high temperatures promote breakage of aliphatic chains and leaching of organic compounds from coal to wastewater. 3.5. Mass distributions of three-phase products The mass distribution and chemical composition of HTD products have attracted considerable attention in industrial
Table 4 Analysis of gaseous products after HTD upgrade. Samples
CO2 (%)
O2 (%)
N2 (%)
CO (%)
H2 (%)
CnHm (%)
H2S (ppm)
HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
40.45 53.19 68.00 71.53 73.03
0.74 0.46 0.45 0.45 0.43
57.92 44.28 28.41 23.33 21.64
0.68 1.50 1.93 1.66 1.60
0.09 0.10 0.26 0.46 0.77
0.11 0.46 0.94 1.55 2.52
765 3580 7110 9682 12,403
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J. Wu et al. / Fuel 148 (2015) 139–144 Table 5 Main cation and anion components of HTD liquid products. Samples
K+ (mg/L)
Na+ (mg/L)
Ca2+ (mg/L)
Mg2+ (mg/L)
SO24 (mg/L)
NO3 (mg/L)
Cl (mg/L)
PO34 (mg/L)
HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
36.9 53.1 74.25 78.65 86.35
821.75 966.25 1364.5 1421.8 1434.5
42.73 60.23 323.5 432.0 761.5
98.03 122.35 124.03 127.50 133.50
570 231 216 176 164
14.0 42.5 61.5 73.0 71.5
11.0 14.0 11.5 13.0 13.5
7.5 6.0 10.0 6.5 7.0
3000 2000
3.0 2.5 2.0
96 1.5 94 1.0
percentage (%)
C-solid C-gas C-liquid
98
4000
percentage (%)
Concentration (mg/L)
100
TOC IC
5000
1000
92 0
200
250
280
300
320
0.5 0.0
90
Temperature (°C )
200 °C
250 °C
280 °C
300 °C
320 °C
temperature
Fig. 3. TOC and IC analysis of HTD liquid products.
Fig. 5. Total carbon balance of solid, liquid, and gaseous products after HTD.
applications. The feed materials included 200 g of raw coal (dry basis; 33.33% of the feed) and 400 g of distilled water (67.67% of the feed). After HTD, the solid and liquid products were separated and then weighed. The treated coals were also dried and weighed, and the mass of gaseous products was determined from the weight difference of feed and the total of solid and liquid products. The mass distribution of solid, liquid, and gaseous products after HTD is shown in Fig. 4. The mass of liquid and gaseous products gradually increased whereas that of solid products decreased as HTD temperature increased. These findings suggest that a fraction of the solid products transforms into liquid or gas products with increasing treatment temperature. The percentage of solid products in HTD-320 was only 27.89%; this value represents a drastic decrease of 16.32% compared with the corresponding value in raw coal. By contrast, the percentages of gaseous and liquid products increased by 1.76% and 3.68%, respectively, compared with baseline values. As previously described, increases in the percentage of released gases, such as CO2 and CH4, may be attributed to the breakdown of
600
liquid gas solid
Mass distribution (g)
500 400 66.67% 67.14% 67.88% 68.37% 69.08% 70.35%
aliphatic hydrocarbons. Increases in the percentage of liquid products may be attributed to leaching of soluble salts and TOC from raw coal. 3.6. Carbon migration during HTD Carbon is the main element in coal that provides the majority of its calorific heat. Given that aliphatic carbons are thermally unstable and form small organic molecules and inorganic gases upon heating, HTD modification causes carbon migration from coal to liquid and solid products. The processing temperature exerts an important influence on carbon migration, as shown in Fig. 5. High temperatures caused significant losses in solid carbon, which leads to increased percentages of gaseous and liquid carbons. In HTD200, for example, minimal amounts of carbon were released into the gas (0.41%) and liquid (0.13%) phases. By comparison, higher temperatures accelerated the decomposition of unstable hydrocarbons in lignite, resulting in 3.5% solid carbon reduction in HTD-320 in contrast to increases of 2.2% and 1.3% in gaseous and liquid carbons, respectively. Carbon contents in solid products remained above 96% within HTD temperatures of 200–320 °C, which indicates that preliminary pyrolysis mildly occurs during HTD upgrade. During hydrolysis, the majority of the carbon is retained and lignite is upgraded by the removal of oxygen-containing groups and volatile hydrocarbons. Disposal of CO2 and treatment of TOCs in wastewater are two issues worthy of further discussion.
300
4. Conclusions 200
0
0.57%
0.80%
1.32%
1.63%
1.76%
100 33.33% 32.29% 31.33% 30.31% 29.29% 27.89% 0
Feed materials HTD-200 HTD-250 HTD-280 HTD-300 HTD-320
Fig. 4. Mass distribution of solid, liquid, and gaseous phase before and after HTD.
HTD, a promising upgrade method for low-rank coals, has recently attracted increased research attention. After HTD modification, significant changes in the properties of XM lignite were observed; these changes included removal inherent water, increases in fixed carbon content, decreases in volatile matter content, and removal of oxygen-containing groups. Such changes resulted in improvements in coal rank.
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C CP/TOSS NMR spectroscopy was used to semi-quantitatively characterize structural changes in lignite before and after HTD modification. Chemical structural changes in lignite agreed well with the properties of the HTD products. The signal intensity of aliphatics decreased whereas that of aromatics increased after HTD. The amounts of oxygen-containing groups, such as carboxyls, hydroxyls, and methoxyls, significantly decreased after HTD, and CO2 and CO comprised the majority of gaseous products formed. Formation of these gases is attributed to decomposition of carboxyl and carbonyl groups, respectively. Evident changes in phenolic hydroxyl content were not observed because of the high bonding energy of this functional group. Solid, liquid, and gaseous products obtained after HTD modification illustrated the mass distribution of carbon: solid matter migrated into the gaseous and liquid products. This phenomenon was especially notable at high temperatures. The proportions of gaseous, liquid, and solid products in HTD-320 were 2.2%, 1.3%, and 96.5%, respectively. The carbon-holding capacity of solid products was as high as 96%. In summary, preliminary pyrolysis mildly occurs during HTD; during this process, carbon contents are maintained and lignite is mainly upgraded by removal of oxygen-containing groups and volatile hydrocarbons. Thus, HTD is an effective method for upgrading low-rank coals extensively. Acknowledgment The authors wish to acknowledge the financial support provided by the National Basic Research Program of China (Grant No. 2010CB227001). References [1] Li WD, Li WF, Liu HF. The resource utilization of algae-preparing coal slurry with algae. Fuel 2010;89:965–70. [2] Katalambula H, Gupta R. Low-grade coals: a review of some prospective upgrading technologies. Energy Fuels 2009;23:3392–405. [3] Fei J, Zhang J, Wang F, Wang J. Synergistic effects on co-pyrolysis of lignite and high-sulfur swelling coal. J Anal Appl Pyrol 2012;95:61–7. [4] Zhan X, Jia J, Zhou Z, Wang F. Influence of blending methods on the cogasification reactivity of petroleum coke and lignite. Energy Convers Manage 2011;52:1810–4. [5] Favas G, Jackson WR. Hydrothermal dewatering of lower rank coals. 2. Effects of coal characteristics for a range of Australian and international coals. Fuel 2003;82:59–69. [6] Favas G, Jackson WR, Marshall M. Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals. Fuel 2003;82:71–9. [7] Mursito AT, Hirajima T, Sasaki K, Kumagai S. The effect of hydrothermal dewatering of Pontianak tropical peat on organics in wastewater and gaseous products. Fuel 2010;89:3934–42. [8] Racovalis L, Hobday MD, Hodges S. Effect of processing conditions on organics in wastewater from hydrothermal dewatering of low-rank coal. Fuel 2002;81:1369–78.
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